docs: def doi_exists(self, doi):
          """
          This method retrieve a boolean according to the existence of a crossref
          DOI number. It returns False if the API results a 404 status code.
  
          args: Crossref DOI id (String)
  
          return: Boolean
  
          Example 1:
              >>> from crossref.restful import Works
              >>> works = Works()
              >>> works.doi_exists('10.1590/S0004-28032013005000001')
              True
  
          Example 2:
              >>> from crossref.restful import Works
              >>> works = Works()
              >>> works.doi_exists('10.1590/S0004-28032013005000001_invalid_doi')
              False
          """
          request_url = build_url_endpoint(
              '/'.join([self.ENDPOINT, doi])
          )
          request_params = {}
  
          result = self.do_http_request(
              'get',
              request_url,
              data=request_params,
              only_headers=True,
              custom_header=str(self.etiquette)
          )
  
          if result.status_code == 404:
              return False
  
          return True